Manchester United chief Sir Alex Ferguson watched his side throw away a two goal half-time lead before rallying to level 3-3 with Swiss outfit Basel.
And Ferguson told Sky Sports after the match: "I think we were careless. It's a wake-up call in many ways. A lack of concentration saw us give the game away really...but we rescued it.
"To concede three goals at home...the defence and midfield have to be better than that in terms of concentration.
"If you lapse, the quality of the Champions League can make you suffer."
Ferguson disputed Basel’s 75th-minute spot-kick which gave them the lead.
The Scot continued: "I don't think it was a penalty. You can see quite clearly that (Antonio) Valencia won the ball. It's a poor decision.
"The referee kept getting in the way of the game. I think he wanted to play. But what can you do?"
On two-goal Welbeck, Ferguson added: "He tired a bit in the second half but it was difficult to take him off...if we'd been in a better position we'd have probably taken him off."